angle of the spark plug to the bore line of cylinder?

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Joe Schlaberdowski

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Well it's all in the subject heading. But then it might have changed from Atlas to Commando as well. Also need (want) to know if it's angled straight out to the side or canted a bit. It's the kind of thing you'd see in a technical drawing.
 
I suppose I could take the head off, then get a bolt with the same threads as the plug and drill a straight hole through that. Then put the bolt where the plug would go then insert a rod down that hole and, using a protractor across the flat of the head, measure the angle made with the protractor. But not being as mechanically inclined as some (or as inclined to take the head off) I'm probably missing something easy as pie. Or, as I was hoping, somebody who knows could just say.
 
A straight edge across the flat of the fins of Both cylinders (since fins are cast and probably thicker near cylinder) is undoubtedly parallel to the line across the cylinder (roughly, but close enough). Though that would not be in a line from rod inserted through spark plug hole, it could be used to check and compensate for straight edge on fins directly under that is on that line. Still, must be something I'm missing. Just revealing my lack of engineering ability.
 
Thanks L.A.B. I'll contact him. To answer "Why?" My son asked me. He has a mill where he works and wanted to drill a bolt at that angle to the center line of the bolt. He wasn't worried about the bolt being rotated or set at the wrong angle. Just wanted the rod to sense TDC without the added inaccuracy or vagueness of touching the piston at an angle since at TDC the ratio of piston movement to crank angle goes to zero anyway. Thought that drilling the bolt at the correct angle would be easy (it is), but what was the angle? Didn't know. Thought I'd ask. BTW, he could just mark the bolt head and the cylinder head and turn the bolt to that for the right position.
 
You could measure off of the machined flat for the rocket spindles and feed. I am pretty sure that surface is perpendicular to the plane of the cylinder head to barrel mating surface.
 
Joe Schlaberdowski said:
Thanks L.A.B. I'll contact him. To answer "Why?" My son asked me. He has a mill where he works and wanted to drill a bolt at that angle to the center line of the bolt. He wasn't worried about the bolt being rotated or set at the wrong angle. Just wanted the rod to sense TDC without the added inaccuracy or vagueness of touching the piston at an angle since at TDC the ratio of piston movement to crank angle goes to zero anyway. Thought that drilling the bolt at the correct angle would be easy (it is), but what was the angle? Didn't know. Thought I'd ask. BTW, he could just mark the bolt head and the cylinder head and turn the bolt to that for the right position.
Click to expand...
If you really want accurate tdc the best way imho is make an engine stop out of an old spark plug,fit a timing disc on the alternator rotor,wind the engine forward with a spanner until it stops make a note of the degree on the timing disc do the same rotating the engine backwards,split the difference and you have exact tdc ,it's an old method you probably already know it but it's foolproof
 
Baz, I think you're right. My son still wants to know. But to get the job done and done, a degree wheel with a piston stop would be quick and accurate.
 
Joe Schlaberdowski said:
Baz, I think you're right. My son still wants to know. But to get the job done and done, a degree wheel with a piston stop would be quick and accurate.
Click to expand...

Just be careful because it is possible to catch a valve on that piston stop. That advise came to me from a very knowledgeable Norton guru.
Pete
 
Rather than a hard stop, consider using a dial indicator with a magnetic base. Most stylus lengths will get you down bore enough, fore and aft of TDC, to set your degree wheel up. You can get the dial indicator stylus almost vertical thus avoid skidding the tip across all the crap and corruption that may or may not be on the crown of your piston.

If you have enough hands (extra help) you could even hold the dial indicator tight fast against something on the cylinder head to take the readings.
 
When you look at the timing given for most Brits , it is usually given as a fraction of an inch, but this distance is to be measured on the cylinder bore , not through the plug hole. Basic geometry about hypotenuse of triangle . With the Commando and Atlas engines there is a complication... the engines are desaxe
that is to say on top dead centre the piston and conrod are not in 90 degree plane to the big end or crankpin... possibly this is why timing was given by the factory in crankshaft degrees...
 
I may be missing the real question, or just not understanding at all without a picture, but surely there is a limitation here in the available hole dimensions, even without a bolt srewed in to the plug hole!

Just freehand, take a (very) thin rod, put it throught the plug hole with piston at TDC or thereabouts, touch it to the piston top, then try to stand the rod 'vertical'.

I don't think you will get it anywhere near vertical before the rod touches both top inner and bottom outer of the plug thread!

Am I totally wrong?
 
oldmikew said:
When you look at the timing given for most Brits , it is usually given as a fraction of an inch, but this distance is to be measured on the cylinder bore , not through the plug hole. Basic geometry about hypotenuse of triangle . With the Commando and Atlas engines there is a complication... the engines are desaxe
that is to say on top dead centre the piston and conrod are not in 90 degree plane to the big end or crankpin... possibly this is why timing was given by the factory in crankshaft degrees...
Click to expand...

This was beaten to death on another thread. The desaxe on a Commando does not amount to a hill of beans with respect to determining degrees TDC from piston depression before and after TDC.

The hypotenuese of triangle is a bit of a moot point when determining degrees TDC from piston depression before and after TDC. One should only be interested in identical readings fore and aft - some use piston stops, some use dial indicators.
 
SteveA said:
Just freehand, take a (very) thin rod, put it throught the plug hole with piston at TDC or thereabouts, touch it to the piston top, then try to stand the rod 'vertical'.

I don't think you will get it anywhere near vertical before the rod touches both top inner and bottom outer of the plug thread!

Am I totally wrong?
Click to expand...

You don't need vertical though near vertical helps to keep the stylus of the dial indicator from traversing the top of the piston too much.

What you are looking for is an identical piston depression before and after TDC to set your degree wheel with. If you measure piston depression before TDC with a slight angle to the stylus, then you use the same angle of stylus to measure piston depression after TDC. The exact magnitude of the measure is not of importance but being identical measures fore and aft is important. I try to measure as much piston movement as possible (consider it the gain) to get an accurate setting of TDC with the degree wheel.
 
Danses with Shrapnel

Many thanks for pointing out the threads.. they are excellent and I was not aware of them. My take on this is that actual tdc will not be halfway btween
two piston readings - does half a degree or so error matter=might have done if you were running very high compression and using alcohol but otherwise probably not.
 
I don't recall the details of the analysis of the impact of desaxe on determining TDC for a Commando but I definitely recall it amounted to a fart in a windstorm for a Commando. In my opinion, 1/2 degree error in the cam timing or ignition timing is not going to amount to much of anything at all. As an example, the factory recommends 28 degrees BTDC for ignition timing whereas one well known tried and true electronic ignition system (Boyer) recommends 31 degrees BTDC; that's a three degree difference!. I am not weighing in on which value is correct, just making a point on sensitivity based on abscence of melted and fragged engines due to a 31 degree advance.

If you have gone to alcohol and/or significantly higher compression ratios then unless you are trying to replicate a previous known build, you are in somewhat uncharted waters and ignition timing should be checked for best performance on a brake dyno, maybe with some other means of checking (plug chops?) and/or top end inspections for detonation.
 
"linear piston travel and advance degrees" Is the name of a thread I started that has a lot of info on the subject of piston positon and crankshaft degrees. I believe the angle of the spark plug hole is 62 degrees.
Dan.
 
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